1. Field of the Invention
This invention relates to a semiconductor optical integrated device, and in particular to a monolithic waveguide-type photodiode optical detector.
2. Description of the Prior Art
In the use of an optical communication receiver having a directional coupler/waveguide as a constitutional element, for example, an optical heterodyne receiver or an optical wavelength division multiplexing detector, it is very important to develop a suitable optical coupling technique to connect a single mode optical waveguide and a photodiode. If it is desired to monolithically integrate a single-mode optical waveguide constituted by a semiconductor together with a waveguide-type photodiode on the same substrate, it is required to form the core layer of the optical waveguide with a material which is different in absorption edge wavelength from a material of the light-absorbing layer of the photodiode so that the optical waveguide exhibits an excellent optical waveguiding performance without causing any serious optical absorption loss, and so that the optical absorption layer of the photodiode has a sufficient degree of absorption coefficient.
One example of the conventional integrated optical device comprising an optical waveguide and a waveguide-type photodiode will be explained with reference to FIG. 1(a), FIG. 1(b) and FIG. 1(c). In FIGS. 1(a)-(c), main manufacturing steps of the integrated device are shown by the cross-section taken along the waveguiding direction of light. First, as shown in FIG. 1(a), crystal layers to constitute a photodiode, i.e. an n.sup.+ -InP buffer layer 2, an n.sup.- -InGaAs light-absorption layer 3 and a p.sup.+ -InP contact layer 4 are grown in the mentioned order all over the upper surface of an n-type InP substrate 1.
Then, as shown in FIG. 1(b), these crystal layers are etched off except for an island region which serves as a photodiode 5. Subsequently, as shown in FIG. 1(c), an n.sup.- -InP lower clad layer 7, an n.sup.- -InGaAsP core layer 8 and an n.sup.- -InP upper clad layer 9 are selectively regrown in the mentioned order only on the optical waveguide region of the n-type InP substrate 1. By employing this fundamental structure, or if desired, by further performing a step of burying the optical waveguide region with InP, the integrated device of the prior art can be obtained after performing ordinary manufacturing steps such as a step of forming electrodes. This conventional integrated device is featured in that the optical waveguide and photodiode are optically coupled through the regrowth interface in the so-called butt-coupling manner.
Another example of the conventional integrated device of an optical waveguide and a waveguide-type photodiode will be explained with reference to FIGS. 2(a)-2(c). In these figures, main manufacturing steps of the integrated devices are shown also by the cross-section taken along the waveguiding direction of light. First, as shown in FIG. 2(a), crystal layers for an optical waveguide, i.e. an n.sup.- -InP lower clad layer 11, an n.sup.- -InGaAsP core layer 12 and an n.sup.- -InP upper clad layer 13, and crystal layers for a photodiode, i.e. an n.sup.- -InGaAs light-absorption layer 14 and a p.sup.+ -InP contact layer 15 are grown in the mentioned order all over the upper surface of an n-type InP substrate 10.
Then, as shown in FIG. 2(b), the n.sup.- -InGaAs light-absorption layer 14 and the p.sup.+ -InP contact layer 15 are etched off except for an island region which serves as a photodiode 16A. Subsequently, the n.sup.- -InP upper clad layer 13 in the optical waveguide region 16B is shaped into a ridge through an spatially selective etching process so as to form an optical waveguide. Then the resultant structure is further processed by the ordinary manufacturing steps such as a step of forming electrodes, thus finally accomplishing the integrated device. This conventional integrated device is featured in that the optical waveguide and photodiode are optically coupled through a so-called evanescent field coupling scheme. These are examples of a photodiode integrated with a single-mode optical waveguide.
According to the conventional integrated device shown in FIG. 1, a regrown interface is introduced at the butt-coupling portion between the optical waveguide and the photodiode, thus giving rise to a large discontinuity in electric field distribution of the guided mode and scattering and reflection of incident light in the vicinity of this regrowth interface, thereby setting forth a problem of lowering coupling efficiency between the optical waveguide and the photodiode. Moreover, since the crystal layers for the photodiode are grown separately from the layers for the optical waveguide, at least two crystal growth processes are required thus complicating the manufacturing process and increasing the number of process steps
In the conventional integrated device shown in FIG. 2, since the optical waveguide and the photodiode are optically coupled through evanescent field coupling, the confinement of light within the light absorbing layer is rather weak, so that in order to attain a sufficient quantum efficiency, a length of several hundreds microns in the optical waveguiding direction is generally required for the photodiode portion. Due to this reason, it is impossible to design a compact device, and the resultant enlargement in the diode area causes increases in both device capacity and dark current, thereby deteriorating the performance of the device.